The present invention relates to a method and an apparatus for replenishing the helium bath of a superconducting generator with liquid helium from a helium reservoir under ambient pressure, the helium bath boiling at subatmospheric pressure and part of the helium changing to the vapor phase while flowing from the reservoir to the bath. The liquid phase of the helium is fed into the helium bath at a distance from the axis of rotation.
In order to attain high reliability in the operation of electrical machines employing rotating superconducting excitation windings, it is considered necessary to feed in the liquid helium in such a manner as to assure that operation of the generator is not interfered with even if there is a malfunction in the cooling system. This "decoupling" of the electrical machine from the cooling system is assured in a simple manner by connecting a reservoir for liquid helium therebetween. The pressure in this reservoir is preferably atmospheric or, in order to prevent impurities in the surrounding atmosphere from contaminating the liquid helium, slightly above atmospheric.
In order to obtain a high current density, the superconducting rotor winding is advisably cooled with helium which boils at a reduced pressure of a few tenths of a bar and thus has a boiling temperature of T&lt;4.2.degree. K. This subatmospheric pressure in the rotor can be maintained in a simple manner by appropriately conducting the exhaust gas stream, taking into account that after absorbing heat, the waste gas leaves the rotor at atmospheric pressure so that additional pumps to produce the subatmospheric pressure in the cold part of the rotor are not necessary.
It has already been proposed, as described by A. Bejan in the work "Improved Thermal Design of the Cryogenic Cooling System for a Superconducting Synchronous Generator", Thesis MIT (1974), and U.S. Pat. No. 4,056,745 to expand the incoming helium through a choke valve to the subatmospheric pressure existing in the rotor, i.e., to effect a Joule-Thomson expansion. The valve must be actively regulated in dependence on the helium stream required in the rotor.
In other proposals, as disclosed in U.S. Pat. Nos. 4,048,529 and 4,082,967, it is assumed that the helium flowing into the rotor, due to thermal losses in the transfer line, has a relatively large vapor phase component. The rotating feeder line is designed in such a manner that the liquid and vapor phases are spatially separated, at least in the radially oriented part of the feeder line. The radial pressure variation in this feeder line is determined by the rotation-dependent compression of the vapor. This line opens into the liquid, which boils at subatmospheric pressure, at that point where its hydraulic pressure is equal to that of the pressure in the vapor column. This replenishing system is self-regulating as long as the vapor phase component does not become too small. In order to assure safe operation even with a large helium flow, it is contemplated to generate the necessary vapor component, if required, by heating the transfer line.
Another method is disclosed in "Cryogenics" 14,429 (1977). Here a liquid is fed through a radial feeder line and is fed into the helium, which boils at subatmospheric pressure, at the point where the hyraulic pressures are equal. One drawback is that the radius at which the feeding takes place is greater than when feeding in a vapor-liquid mixture. For a 50 Hz rotor, for example, this radius must be larger than 0.33 m. A further drawback is that only pure liquid is conveyed through the radial feeder line.